RNA synthesis in fractionated rabbit bone marrow erythroid cells will be studied to determine the absolute rate of synthesis of ribosomal, messenger and transfer RNA in order to assess the contribution of nuclear RNA synthesis for specific globin and other messenger RNA precursors. Specific messenger RNAs will be determined by assaying specific sequences of these RNAs. The size of nuclear RNA messenger RNA precursors will be determined in order to develop a model for transcription in differentiated cells. Erythropoietin stimulation of globin mRNA synthesis will also be studied.